1. Field of the Invention
The acceptability of polyunsaturated oils in many commercial applications, particularly in the food industry, requires the reduction of the polyenes to a more stable monoene or diene stage. For example, the stability of soybean oil as it relates to flavor reversion can be vastly improved by eliminating the linolenate, which exists at a level of about 6 to 9% in the natural oil. This is preferably accomplished by selectively hydrogenating the oil in order to reduce the proportion of linolenate without a concurrent increase in the level of saturates. In a commerical operation, the ideal catalyst for promoting such hydrogenations is one which is not only selective, but also highly active so as to minimize the duration of the reaction. This invention relates to a novel catalyst system for the rapid, partial hydrogenation of polyunsaturated oils.
2. Description of the Prior Art
Two basic catalytic systems have developed widespread acceptance in the selective hydrogenation of natural oils: insoluble catalysts and homogeneous catalysts. Insoluble types include those which have been supported on an inert carrier by adsorption, ion exchange, or precipitation, as well as those which have been coprecipitated with a suitable agent. Exemplary supported catalysts are taught in de Jonge et al., U.S. Pat. No. 3,278,568 wherein elemental copper, or copper compounds are dispersed on a variety of carriers including activated clay, diatomaceous earth, silica gel, and several metallic oxides. Koritala [J. Amer. Oil Chem. Soc. 45(3): 197-200 (1968)] and U.S. Pat. No. 3,515,678 teach supporting copper catalysts by precipitation onto micronized silica or alumina, and by exchange onto molecular sieve zeolites. In U.S. Pat. No. 3,749,681, Koritala shows adsorbing copper ammonium complex onto a microspheroidal silica gel. A coprecipitated catalyst is disclosed in Koritala, U.S. Pat. No. 4,174,300 wherein sodium silicate is reacted with a water-soluble copper salt to form a precipitate which is subsequently vacuum dried and heat treated. While insoluble catalysts tend to be characterized by high selectivities, their activities are often insufficient for practical commercial application.
The potential of soluble or homogeneous catalysts in the hydrogenation of polyunsaturated oils has been extensively studied in recent years. By virtue of being in the same phase as the oil, they have the inherent advantage of possessing more activity than their insoluble counterparts. While there are many types of homogeneous catalysts, one which is acceptably selective and moderately active is the Ziegler-type catalyst as described by R. Stern et al. in U.S. Pat. No. 4,038,295. These are composed of transition metal salts in combination with a reducing agent, as exemplified by copper stearate-triethylaluminum.